Surface treatment of steels



Jan. e, 1970 l. L. NEwE-LL l 3,488,233

SURFACE TREATMENT OF STEELS Filed July 27; 1967 PM .fl m um W my .W XW-W as mg Q u Q Q Q United States Patent O M 3,488,233 SURFACE TREATMENTOF STEELS Isaac L. Newell, Wethersfield, Conn., assignor to UnitedAircraft Corporation, East Hartford, Conn., a corporation of DelawareFiled July 27, 1967, Ser. No. 656,531 Int. Cl. C21d 1/46 U.S. Cl. 14S-15.5 8 Claims ABSTRACT OF THE DISCLOSURE A process for carburizingmaterials, such as the ferrous metals, with or without the formation ofa protective coating, by simple immersion in a molten salt bathcontaining lithium carbonate as the active ingredient.

BACKGROUND OF THE INVENTION It is known that the carburization ofvarious materials, particularly the ferrous base alloys, may beconveniently effected by simple immersion in molten salt baths.Exemplary of some of the various techniques previously employed in theart for this purpose are the processes set forth in the patents toLeininger 2,492,803 and 2,568,680, and Freudenberg 1,796,248. In mostprocesses of this type wherein the desired carburization is performed bysimple immersion and without the application of an externalelectromotive force, the usual carburizing ingredients utilized includethe various poisonous cyanides or suspended free carbon. A furthercharacteristic of these prior art techniques is the use of an elevatedtreatment temperature, typically 15001`800 F.

SUMMARY OF THE INVENTION The present invention relates to the surfacetreatment of materials, particularly the ferrous metals, in molten saltbaths. It contemplates the use of lithium carbonate as the activecarburizing ingredients in such baths, with or without the inclusion ofadditional salts or other ingredients to adjust the melting point of thecomposition and control the rate and character of the carburization,including such ingredients which are commonly identified as catalysts,activators and inhibitors. It further contemplates a technique wherebycarburization of the underlying substrate may be effected without thebuildup of a surface layer on the part, or carburization with theconcurrent formation of a corrosion resistant coating on the part,depending upon the particular requirements sought in connection with thefinished article.

BRIEF DESCRIPTION OF THE DRAWING The drawing comprises a graph ofhardness versus depth of penetration for several samples of low carbonsteel carburized by the process of the instant invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS As previously indicated, moltenlithium carbonate is the active carburizing ingredient present in thetreatment bath of the present invention. In various tests conductedutilizing this process, parts have been carburized in melt compositionwhich have been varied from 100 percent lithium carbonate to those inwhich the lithium carbonate was present in amounts as low as 20 molpercent, with equally good results in all cases. Several additives havebeen included in the bath compositions for various reasons and, in amore preferred composition, lithium fluoride was added and, in fact,comprised the predominant element in the bath on a molar basis. Thisaddition was made solely to lower the melting point of the bath andexperience has demonstrated that the lithium fluoride appears to provideno other function as far as the carburization process is concerned. Thepreferred composition incorporating lithium fluoride was found toprovide effective carburization at a temperature of about 1418 F. Anexample of such a bath would be one containing about 20 to 99 molpercent lithium carbonate with the balance a lithium halide.

In another preferred bath composition boron oxide was also added to thebath, primarily to prevent the formation of a coating buildup on thesurface of the part being treated in those instances where the coatingwas considered undesirable. The boron oxide was found to be effective inthis regard without significantly interfering with the basic substratecarburization. In some embodiments of the present invention, therefore,the boron oxide addition will be preferred. Other activating agents orcoating inhibitors which also appear satisfactory include sodiumtetraborate, silicate and the various alkali metal silicates. It isapparent, on the other hand, that if the formation of acorrosion-resistant coating is desired on the finished product, boronoxide or the other comparable substances should be substantiallyeliminated from the melt. Although the boron oxide concentration doesnot appear particularly critical and substantial `quantities appeartolerable in the melt, the concentration utilized should not be so highas to interfere with the basic carburization process. Accordingly,concentration of about 3 mol percent boron oxide will normally bepreferred when carburization alone is desired. A suitable bath wouldconsist essentially of about 20 to 99 mol percent lithium carbonate, upto mol percent lithium iiuoride and 1 to 5 mol percent of a coatinginhibitor. A further example of a suitable bath would be one containing20 to 50 mol percent lithium carbonate, 50 to 80 mol percent lithiumliuoride and 1 to 5 mol percent boron oxide.

A fourth ingredient included in a more preferred composition is nickelchloride which was added to increase the rate of reaction and, hence,exhibits a catalytic function. It appears to perform this functionalthough present in a very minor amount, a concentration ofapproximately 200 parts per million having been demonstrated assatisfactory in this regard.

The present invention is described in further detail in the specificexamples which follow. It will be understood that these examples areincluded for the purposes of illustration only and no limitation isintended or should be implied therefrom.

EXAMPLE I A carbon steel rod, formed from SAE 1010 material, having anominal composition, by weight, of

Percent Carbon .08. 13 Manganese .30-.60 Phosphorous max-- .04 Sulfurmax .05 Silicon max .10 Iron remainder was immersed in a molten bath oflithium carbonate at 1400 F. for a period of 21 hours. At the end ofthis time the rod was quenched in water. The rod was sectioned andexamined microscopically, examination revealing a uniform coatingapproximately 0.001 inch in thickness. Analysis by X-ray dilfractionshowed this coating to consist of lithium ferrate (-LiFeOz), andchemical analysis of the steel showed a carbon content increase in thesurface layer to 0.25 percent by weight.

EXAMPLE Il A sample identical to that of Example I was treated undersubstantially identical conditions, except that a blanket of argon was'maintained over the bath during the treatment-period. -v The-rodez wassubsequently-foundto have a coating similar to that found in Example land, in addition, the surface carbon content of the steel was increasedto 0.36 percent by weight.'y r H f EXAMPLE 111 a A rod of SAE 1010 lowcarbon steel was immersed in Samples formed of SAE 1010 steeltubing in14 inch l lengths, cut from the same stock, were thoroughly cleanedutilizing emery'cloth abrasion followed by immersion'for 5 minutes in abath of 30 percent hydrochloric acid.

The samples were immersed in a salt bath prepared to the followingcomposition:

Mols Weight, gms.

Ingredient:

Lithium fluoride 60 445 Lithium carbonate- 422 Boron oxide 5. 100 Nickelchloride l 1 37 l The nickel chloride was held in a hard vacuum for 3days at 160 F. before addition to the melt.

A variety of treatment conditions were utilized for the samples assummarized in the following table:

Carbon Average Sample test condition VHN content Sample:

A Annealed, untreated 103 11 D Immersed in molten salt at 1,400 F. 708.36

for 22 hours, argon blanket, water 4 quenched. l 13--... Heated to1,400F., argon blanket, 192 .10

` water quenched. n p 0...-. Immersed in l-week'old molten salt at 405.14

. 1,400 F.,tor 20 hours, argon blanket, n v y water quenched. y E....-immersed in 2-week'old molten salt at 374 1.-16

1,400" F. for 20 hours, @5% C02 in argon blanket,.water quenched.11...-- Same as (E), section 1 inch above im- 93 02 v mersed portion.

The results of the `various treatments as analyzed metallurgically areillustrated in the drawing, the sample number referred to in thepreceding table corresponding t'o that utilized in the drawing.

It is evident from the tests conducted that the hardness, depth ofhardening, and extent of the corrosionresistant coating are functions ofthe process parameters primarily time, temperature and meltcompositionalthough the bath condition and article pretreatment are of importance.Treatment temperatures as low as 12`O0l F. appear to provide the desiredcarburization. l l

The corrosion-resistant coating, when formed, appears to compriselithium ferrate. While it is not certain how the reactions occur in theformation of this coating, it is believed that decomposition ofthelithium carbonate takes place with a subsequent, formation of vaconversion coating withA` the iron and at the same time reduction "tol.at-carbide, -according to the following equation:V

It may readily be seen that substantial regeneration of the bath can beeffected by treatment with carbon dioxide to reform the lithiumcarbonate.

Samples from the product of Examples I and 'VII were tested in a saltspray box for a period` of 24 hours. While no corrosion was noted on thecoatedareas, those regions wherein the base metal was exposed by cuttingaway the coating were badly corroded. Similarly, a humidity test at 120F. for 24 hours and atotal immersion test for 72 hours at 70 F.A showedno corrosion in the coated areas. I# 1', v v v .From the foregoingdescriptionvit w ill readily be seen that there has been'provided bythis invention means for carburizing materials in asimple immersionprocess lcharacterized by the absence of toxic salts. Further, theprocess is inherently flexible in that the car'burization may beeffected with or without the formation of a protective coating on thefinished article bythe, simple adjustment of thebath'compo'sition.Alternatively, therefore, the process in one form 'may be characterizedas a coating process, the coated article produced displaying excellentcorrosionresistance characteristics.

What is claimed is:

1. A process for carburizing the surface of the ferrous metals whichcomprises immersing the metal in a molten salt Ibath containing' fromabout 20-100Y percent lithium carbonate as the active carburizingingredient, maintaining the bath at a temperature of at least 1200 F.,and continuing the' immersion until the desired carburizng treatment iselected.

2. The process according to claim 1 wherein the molten salt bathconsists essentially of about 100 percent lithium carbonate. v

3. The process -according to claim 1 wherein the molten salt bathconsists essentially of about 20-99 mol percent lithium carbonate,balance a lithium halide.

I 4. The process according to claim 3 wherein the lithium halide islithium iiuoride.

y 5. The process according `to claim 1 wherein the molten salt bathconsists essentially of about 20-99 mol percent lithium carbonate, up tomol percent lithium iiuoridc, and 1-5 mol percent of a coatinginhibitor. l "6. VThe process of claim 5 vwherein the coating inhibitois boron oxide.' I v v 7.A process for carburizing the ferrous'metalswithoutthe formation of a coatingfthereon which comprises immersing'themetal in a molten salt bath consisting essentially of 20,-50 mol percentlithium carbonate, 50-80'mol percent vlithium uoride, andA 1'-5 molpercent boronoxide,

1 maintaining the bath ata temperature of at least 'l200 F.,

References Cited UNITED STATESv PATENTS 2,049,806 8/1936 Holt l48-15.5y2,568,860 9/1951 Leininger et al 14S-15.5I

CHARLES LOVELL, Primary Examiner y ,I Us. C1. X.R. 14s-6.11, 2o, 31.5,39

